1. Field of the Invention
The present invention relates to the field of waveband separating prisms. More specifically, the invention relates to prism systems that separate an incoming beam of broad spectrum light into two or more outgoing beams, where each of the outgoing beams is either parallel or perpendicular to the direction of the incoming beam. Each of the outgoing beams encompasses a waveband that is a subset of the original incoming beam.
2. Description of Related Art
Waveband separating beamsplitters have long been used in television and video cameras in order to efficiently split visible white light into red, green, and blue components that are directed to three separate sensors. This approach maximizes both throughput and resolution. Waveband separating beamsplitters are also widely used in multi-panel projection systems, although in such applications the beamsplitter is actually used as a beam combiner to combine the narrow band light from two or more object panels into a single beam of broadband light.
A conceptually simple method of splitting a broadband beam of light into multiple beams of narrower bands of light is achieved through the use of what is referred to as a “cross dichroic waveband splitting prism cube,” or “X Splitter,” or sometimes as a “Cube Color Splitter.” FIG. 1 depicts this type of beam splitter. (Although it is most often used for color combining, it may also serve equally well as a color separating prism by simply reversing the roles of the incoming and outgoing beams.) The cross dichroic beam splitting prism is frequently used in LCD projectors for combining light from three differently colored image panels. This prism system has dichroic surfaces (or mirrors) inclined at a 45 degree angle to the incoming beam. When used in applications such as an LCD projector, where the light is predominantly polarized, a relatively steep angle of 45 degrees is suitable. However, in applications where the incoming light beam consists of non-polarized light, the prism will suffer from negative polarization effects. In particular, the beamsplitting efficiency will vary with respect to s-polarized and p-polarized light. Interference will be created at this surface and can destroy the color purity of the device.
It has been reported that dichroic coatings exist which overcome this interference at angles of 45 degrees. However, these coatings require the application of up to 77 layers of film. A preferred means of splitting beams composed of non-polarized light would be to use a dichroic surface at a much shallower angle of around 15 degrees. As such angles, the cross dichroic prism would not function.
The beamsplitter of choice for most three-sensor camera systems is illustrated in FIG. 2, and was first disclosed by d Lang et al. in U.S. Pat. No. 3,202,039. An alternative to the cross dichroic beam splitting prism cube is the well-known Total Internal Reflection (TIR) beamsplitting prism. The advantage of the TIR beamsplitter is that the dichroic surfaces are inclined at a very shallow angle with respect to the incoming beam. Numerous modifications have appeared in the patent literature over the years, including those disclosed in U.S. Pat. Nos. 4,009,941, 4,035,836, 4,084,180, 4,784,496, 5,644,432, and 6,144,498. Two, four, and five channel variations are known of this basic prism type.
A drawback to this method is that the three output beams of the three-channel TIR beamsplitters are not oriented parallel to the direction of the incoming beam, thereby making them unfeasible for use in applications such as a periscope, which requires the system to be inserted into a cylindrical housing sleeve.
Therefore, there is a need for a compact waveband separating prism system where each of the outgoing beams is either parallel or perpendicular to the direction of the incoming beam. Such a beamsplitting prism would have wide-ranging applications in cameras, projectors, and optical systems requiring multiple wavebands.